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The strong perpendicular magnetic anisotropy of L10-ordered FePt has been the subject of extensive studies for a long time. However, it is not known which element, Fe or Pt, mainly contributes to the magnetic anisotropy energy. We have investigated the anisotropy of the orbital magnetic moments of Fe 3d and Pt 5d electrons in L10-ordered FePt thin films by Fe and Pt L2,3-edge x-ray magnetic circular dichroism (XMCD) measurements for samples with various degrees of long-range chemical order S. Fe L2,3-edge XMCD showed that the orbital magnetic moment was larger when the magnetic field was applied perpendicular to the film than parallel to it and that the anisotropy of the orbital magnetic moment increased with S. Pt L2,3-edge XMCD also showed that the orbital magnetic moment was smaller when the magnetic field was applied perpendicular to the film than parallel to it, opposite to the Fe L2,3-edge XMCD results although the anisotropy of the orbital magnetic moment increases with S like the Fe edge. These results are qualitatively consistent with the first-principles calculation by Solovyev et al. Phys. Rev. B 52, 13419 (1995), which also predicts the dominant contributions of Pt 5d to the magnetic anisotropy energy rather than Fe 3d due to the strong spin-orbit coupling and the small spin splitting of the Pt 5d bands in L10-ordered FePt.
Ikeda et al. (Mon,) studied this question.